This invention relates to sensory aids for the blind and in particular to aids of the binaural type assuming the form of spectacles.
Binaural sensory aids for the blind have been described in U.S. Pat. Nos. 3,172,075 and 3,366,922. In the first patent a single transmitter transducer is mounted in the vicinity of the bridge of a spectacle-like frame, and two receiving transducers are mounted one on each side of the transmitting transducer. The electronics are accommodated in a remote unit carried in the user's pocket, for example, although the above specifications refer to the possibility of including receiver pre-amplifiers in the spectacle frames. In this system the transmitted signal as reflected by objects in its path is received by two spaced apart receiver transducers to provide directional and range information on the source of the transmitted signal reflection. The transmitted signal is frequency swept and accordingly the received echos will show a difference in frequency with respect to the transmitted signal at any one instant due to the time delay of the signal paths. The received signals are therefore each multiplied with the transmitted carrier to produce audible beat frequency signals. The respective frequencies of which are proportional to the distance between the particular receiver transducer concerned and the source of reflection. The beat frequencies will be the same when the source of reflection is directly in front of the user. In U.S. Pat. No. 3,366,922 resolution of direction is improved by providing the receiving transducers with divergent polar lobes so that directional information is supplied by the differing relative levels of the two received signals thus as with a human auditory system. Since the gain of the two receiver channels is made variable to allow the user to select the volume of the signals perceived it is essential to be able to control the gains of each receiver channel in precise synchronism. It is desirable in such spectacle type aids to be able to fold the side frames down as in the manner of ordinary spectacles, and to be able to use a light flexible cable between the remote unit and the spectacle frame which may be detachable from the frame. In the known aids these requirements are not easily met owing to the large number of conductors in the cable between the remote unit and the spectacle frame and also running around the frame temples and the frame proper.
An obvious step towards meeting the above mentioned desiderata which is now technologically possible, is to mount the complete receiver circuits inside the spectacle frame. However, it is still necessary to control the gain of the aid from the remote unit and to achieve precise gain tracking of both left and right channels. Thus, a relatively large number of conductors would still be required between the remote unit and the spectacle frame.